Field of the Invention
This invention relates to touch input devices, for example for use in display devices with touch screens.
Description of the Related Art
Touch screens are becoming increasingly common in consumer electronics applications where an LCD display is present in a device e.g. mobile phone, PDA or camera. User interaction via a touch screen saves the space required for key inputs and therefore allows a larger display area for a given size of device. The touch screen provides a 2D position sensing function, and it can be used generally as a means of controlling or interacting with devices.
Of the possible physical effects used to locate the “touched” position on such a screen, sensing the capacitance change induced between orthogonal sets of electrodes, or between a grounded stylus and individual electrodes, promises the highest resolution whilst integrating most easily with existing manufacturing processes.
Typically the electrodes of a high resolution 2D capacitance sensor are laid out in a matrix pattern of intersecting orthogonal electrodes, indicated as electrodes 10a and 10b in FIG. 1. The electrodes may be formed using two isolated layers of a transparent conducting material such as indium tin oxide (ITO). As the object moves over the electrodes, the capacitance between the electrodes and the object and the capacitance between the electrodes varies. Sensing circuits which connect to the electrodes are able to detect changes in these capacitances which can then be interpreted to determine the position of the object.
Typically position sensors are combined with displays in the form of an overlay providing touch or stylus input. Sensors based on capacitance sensing consist of sets of electrodes which are connected to drive and/or sensing circuits. The location of an object, for example a stylus or a finger, is detected by measuring changes in the capacitances associated with the electrodes and the object.
In FIG. 1, the electrodes are shown as narrow lines, however the outline of the electrodes may be varied depending on the detailed operation of the sensor. For example in order to increase the capacitances between the sense electrodes and the object it may be preferable to use wider electrodes for example as shown in FIG. 2.
In this case, the electrodes consist of diamond shapes which are joined at their vertices to form horizontal and vertical sense electrodes.
The electrodes are in the form of straight electrode lines 20a,20b, with enlarged diamond shaped portions 22a,22b along the lines. The pitch of the diamonds 22a,22b (i.e. the distance between the diamond centers) corresponds to the pitch of the electrode lines of the other array, so that a regular array is defined.
The area presented by the electrodes is substantially increased compared to FIG. 1 resulting in higher capacitance values which can be more easily measured.
In the case where the sensor is combined with a matrix display device, the number of sense electrodes is likely to be lower than the number of rows and columns of pixels within the display but interpolation techniques can be used to determine the position of the object when it lies at intermediate positions between the centers of the sense electrodes.
A concern that arises when locating sense electrode structures in the optical path of a matrix display device is that the pattern of the sense electrodes may be visible as a variation of brightness over the surface of the display. For example, a conducting layer of ITO might typically have a transmission of 95%. Brightness variations of only 1% can be seen by the eye particularly when they have a linear or repetitive structure making it likely that under some circumstances the electrode pattern will be visible to the person viewing the display. The presence of the sense electrodes may therefore degrade the quality of the displayed images particularly when moving images are being viewed.
A further concern is that when the object to be sensed is significantly smaller than the sense electrode pitch, this will affect the way in which the capacitance values change with the position of the object, making it difficult to uniquely locate the position of the object when it is centered on one of the sense electrodes.
For example, FIG. 3 shows in more detail part of the electrode layout and the corresponding cross section is shown in FIG. 4.
FIG. 3 shows a line X-X along the center of one of the electrode rows. When the stylus 40 is located at the center of the line X-X as indicated in FIG. 4 (i.e. at the middle of one of the diamonds in the row direction electrodes 30b,32b), it will have a relatively large effect on the capacitances associated with the row direction sense electrodes 30b,32b (these will be termed B electrodes in the following description) but a much smaller effect on the capacitances associated with the adjacent column electrodes 30a,32a (these will be termed A electrodes in the following description). This may make it difficult to detect the location of the stylus on one set of electrodes, for example the A electrodes, when the stylus is centered over one of the other set of electrodes, for example the B electrodes. In particular, from this starting point, movement of the stylus along the column direction has much less effect on the capacitance than movement of the stylus along the row direction.
This is illustrated graphically by FIG. 5 which shows an estimate of the capacitance between a stylus and the sense electrodes when moving either side of the center of the line X-X. Curve 50 represents the capacitance between the stylus and the B (row) electrode and the curves 52 and 54 represent the capacitance between the stylus and the two A (column) electrodes to either side.
For the graph of FIG. 5, it is assumed that the stylus 40 has a tip diameter of 1 mm and the diamond shapes of the sense electrode arrangements have a side with a length of 4.2 mm (this is dimension L shown in FIG. 3).
In FIG. 5, the x-axis shows the position along the line X-X. Position 0 corresponds to the center of a diamond 32b (as shown in FIG. 4). Thus, this position corresponds to the maximum capacitance to the row direction sense electrodes 30b,32b. When moving to the side, the capacitance to the row direction sense electrode drops (curve 50), but the capacitance one of the column direction sense electrodes increases (curves 52 and 54).
It can be seen that when the stylus 40 is centered on the line X-X, the capacitance between the stylus and the adjacent A electrodes falls to a low level as most of the electric field lines between the stylus and the sense electrode terminate on the B sense electrode. This will make it difficult to detect which of the A electrodes the object is closest to.
In general, the way in which the capacitances associated with the sense electrodes vary with the position of the object depends on the dimensions and the shape of the sense electrodes. However, the electrode shape required to produce the desired sensor characteristics may not be consistent with the pattern required to minimize the visibility of the sense electrodes. Reducing the visibility of the electrodes is particularly important when the sensor is combined with a display device.